260 PRACTICAL ORGANIC CHEMISTRY saturated compound, forms additive compounds with halogens and halogen acids. With permanganate solution it may be converted into glycerol, CH2:CH.CHoOH + H\jO -\- O = CH2OH.CHOH.CH.jOH. On oxidation with silver oxide it yields the corresponding alde- hyde (acrolein) and the acid (acrylic acid). PREPARATION 31 Isopropyl Iodide.—The replacement of hydroxyl by iodine in the action of phosphorus and iodine on alcohols has already been described (see Prep. 6, p. 68), but here the presence of an excess of hydriodic acid, which is due to the action of water on the phosphorus iodide, PI8 -h 3H,0 = P(OH)3 + 3HI, exerts in addition a reducing action on certain of the hydroxyl groups. By diminishing the proportion of phosphorus and iodine to glycerol, the reaction may be interrupted at an earlier stage, when ally! iodide is formed. This is probably due to the splitting off of iodine from propenyl tri-iodide, CILJ.CIILCIU = CH2:CH.CH.J + I3, On the other hand a larger proportion of phosphorus and iodine or cone, hydriodic acid will reduce allyl iodide to propylene, CII3:CM.CHaI -1- HI = CH2:CII.CH, + L,. The action of hydriodic acid on glycerol is typical of the polyhydric alcohols. Hydriodic acid converts erythritol into secondary butyl iodide, and mannitol into secondary hexyl iodide. The normal iodides are never formed. PREPARATION 32. Bpichlorhydrin.—It is a noteworthy fact that althoug-h hydrochloric acid can replace hydroxyl by chlorine in the case -of the monohydric alcohols, the number of hydroxyl groups wliich are substituted in the case of polyhydric alcohols is strictly limited. Like glycerol, ethylene glycol gives a chlorhydrin, CHoOH.CHoOH -1- IIC1 = CH-oOH.CHoCl + I-IflO.